Morphology, crystallization and rigid amorphous fraction in PDMS adsorbed onto carbon nanotubes and graphite

Abstract

Morphology and thermal transitions of low-molecular weight polydimethylsiloxane (PDMS) adsorbed onto carbon nanotubes (CNTs) and micrometric colloidal graphite sheets (CG) were studied employing SEM, isothermal nitrogen adsorption–desorption and calorimetry techniques. The CNTs were found to agglomerate forming voids between tubes of a broad range, while adsorption of the polymer from a solution results in the expected wrapping of CNTs by PDMS chains and, further, in filling of voids, as documented by SEM and by reduction in specific surface area, SBET. By employing three different thermal protocols in differential scanning calorimetry (DSC) measurements, namely, fast cooling, slow cooling and isothermal annealing of crystallization, direct and indirect filler effects on PDMS mobility (glass transition temperature, Tg, mobile amorphous fraction, MAF), crystallization temperature, Tc, and crystalline fraction (CF) were followed. Results were further evaluated in terms of the rigid amorphous fraction, RAF, and the respective contributions of crystals, RAFcrystal, and of filler-polymer interfaces, RAFfiller. CNTs were found to increase the rate and degree of crystallization of PDMS, while RAF seems to correlate with a previously proposed fraction of polymer ordered at the interface with CNTs for the uncrystallized samples and with CF for the semicrystalline samples. CG particles interact at a lesser extent with PDMS (less RAFfiller), while their presence results in suppression of rate and degree of crystallization (i.e. opposite effects than those of CNTs), so that contributions to RAF could be separated in that case. For both cases of filler, changes in RAF correlate well with those in the SBET value of initial particles. This result supports previous observations in nanocomposites based on PDMS of various low molecular weights adsorbed onto metal oxide nanoparticles with a wide range of SBET.